The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical polarization splitters (dividers) or combiners (multiplexers).
In fiber optical transmission systems the light beams traveling in two fibers must often be combined into a single fiber, a device which accomplishes this is called a combiner or multiplexer. Similarly, in such systems one beam must frequently be split into two or more beams, a device which accomplishes this is called a splitter or divider. A splitter or combiner is simply the same device used xe2x80x9cin reversexe2x80x9d i.e. when a beam is launched from a single fiber through a splitter/combiner the beam will be split into two beams and directed to two output fibers, if two beams are launched from the previous xe2x80x9coutputxe2x80x9d (now input) fibers back through the splitter/combiner the beams will be combined into a single beams and directed to the single xe2x80x9cinputxe2x80x9d (now output) fiber as such splitter/combiners may be referred to as an optical xe2x80x9ccouplerxe2x80x9d.
However the use of standard splitter/combiners brings with it an important consideration: each splitting or combining of the beam causes a 3 dB loss of light power, as a beam traveling through a fiber optic transmission system may undergo many splittings and combinations the cumulative effect of the 3 dB losses can be very large.
One practicable splitter/combiner which does not incur the 3 dB losses is a so called polarization splitter/combiner in which an incoming beam having two orthogonal linear polarizations is split into two beams by being passed through a birefringent displacer or by the use of a polarizing beam splitter (either a prism cube or a thin film on a glass plate) with one beam having a first linear polarization state and the other a second linear polarization state with the first and second polarization states being orthogonal to each other. When used as a combiner a beam of a first linear polarization state from one input fiber is combined with a beam of second linear polarization state from a second input fiber into a third output fiber carrying both beams.
Previous polarization splitter/combiners relying on the use of polarizing beam splitters and birefringent displacers have required three lenses to couple light into the input and output fibers. The present invention is directed to an optical polarization splitter combiner of the xe2x80x9cinlinexe2x80x9d type which provides a more compact coupler by using a reflector to reflect one of the beams back through the polarizing beam splitting cube (PBSC) or birefringent displacer. This has the advantage of using one collimating lens per two input or output fibers with all of the input and output fibers lying parallel to each other. The reduction in component count also greatly simplifies the necessary alignment of the components and thus reduces size and cost. The present invention also provides for the ready alignment of the components of the splitter/combiner to reduce insertion loss thus further saving cost and complexity.
An inline fiber optic polarization combiner/divider is proposed for use in fiber optic communications. It utilizes a polarizing beam splitting cube (PBSC) or a birefringent displacer placed between a pair of lenses. A reflecting film (either external or directly applied to one of the faces of the PBSC or birefringent displacer) is used to completely reflect one of the two polarizations and is positioned in the such that the optical path length between the lens and the reflecting film is equal to one focal length in order to accomplish an optical path where the input and output optical fibers are in-line with the package.
As a polarization splitter, the input optical signal is input through a polarization maintaining (PM) fiber or a single mode fiber (non-PM). The output optical signals are fed into two PM or non-PM optical fibers, one is placed next to the input fiber and the other is aligned on the other side of the PBSC or birefringent displacer. As a polarization combiner, the two input optical signals (with orthogonal polarizations) are input through two polarization maintaining (PM) fibers, one is placed next to the output fiber on one end of the PBSC or birefringent displacer and the other is aligned on the other side of the PBSC or birefringent displacer. The combined output optical signal is fed into an output fiber which can be either PM or single mode (non-PM). The common port fiber (the input fiber in a polarization divider operation or the output fiber in polarization combiner operation) can be a polarization maintaining fiber or a single mode fiber (non-polarization maintaining fiber) depending on the application requirement.